This invention relates to metallic containers and has particular reference to hermetically double seaming metal end closures to very thin-walled metallic can bodies, for example those used for packaging beer and carbonated beverages, and the like.
The conventional method of attaching metal end closures to open-ended metallic container bodies includes, flanging the marginal end portion of the can body, and double seaming the body flange with the closure flange by rolling or tucking the closure flange under the body flange to form five metal layers in the seam area, and then pressing or ironing the five layers tightly together against the container body wall.
Heretofore, hermetic, pressure-resistant double seams between metal end closures and very thin-walled metallic container bodies were very difficult to obtain and were seldom obtained. The main reason was that forming conventional double seams required forming a 0.090 to 0.100 inch conventional flange of the marginal end portions of the container body walls. While this presents no problem in relation to container walls of conventional thickness, that is, greater than about 0.008 inch adjacent the open ends of the container bodies, it was found that certain very thin, highly-worked metal body walls having a substantially vertical or axial grain direction and whose thickness adjacent their marginal open end portions was less than about 0.006 inch for three-piece containers and less than about 0.0057 for two-piece containers could not be flanged and hence could not readily be double seamed. The problem is that the metal of such highly-worked highly-directional body walls often will not elongate to the extent required for forming a flange. The wall metal at the marginal end portions is too thin, and too brittle, and elongation during flanging is in the same grain direction as in the drawing and ironing process. This inability to elongate as required produces cracks in the flanges and such flanges often preclude obtaining hermetic pressure-resistant double seams.
Heretofore, can manufacturers have sought to overcome flange cracking and resulting inhermetic seams by neckingin or providing extra metal in certain end regions of can body walls for example drawn and ironed containers by beefing up the thickness of the metal in the area to be flanged about 0.002 inch or more and by using sealant materials in the double seams. But neither remedy prevents cracks in flanges and each is costly and brings on its own problems. For example, providing extra metal at end regions of drawn and ironed container bodies makes it difficult to strip them from their drawing punches.
Even if uncracked flanges satisfactory for double seaming could be obtained in these very thin highly-worked container bodies, the edges would be excessively sharp and cause damage to compound linings required on the interior surfaces of end closure flanges, and to sealing rubber gaskets of can testing and can filling machines. Also, flanged very thin highly-worked metal container bodies are usually weak at their upper regions and are highly susceptible to denting, crushing and other abuse during storage and handling.
It has now been found that by employing a body curl instead of a flange, the aforementioned flange-related problems are greatly reduced and unexpectedly hermetic double seams resistant to product internal pressures can be obtained. Body curls have been found to require less metal elongation than conventional flanges. Whereas the latter requires 7 to 8% elongation, the former requires less than about 4% elongation. Because of this, body curls formed of extremely thin highly-worked metal containers rarely have cracks or puckers.
Double seaming curled rather than flanged container body walls saves metal because end closure flanges can be shorter than conventional ones.
In view of the above shortcomings of flanging and the above and other advantages of curling the marginal end portions of very thin highly-worked metal container bodies, it is an object of this invention to provide a very thin, highly-worked metallic cylindrical container that has a metal end closure hermetically, and pressure resistantly double seamed thereto.
Another object of this invention is to provide double seamed cylindrical containers of the aforementioned type wherein the container body wall metal has a substantially axial grain direction and its thickness is less than 0.006 inch at its marginal end portion for three-piece and less than about 0.0057 inch for two-piece containers.
Another object of this invention is to provide the aforementioned double seamed containers wherein before double seaming their marginal end portions were in the form of body curls.
Another object of this invention is to provide the aforementioned double seamed containers wherein the metal layers of the double seam include a radially compressed body curl.
Another object of this invention is to provide a method of hermetically double seaming a metal end closure to a very thin walled container body.
It is another object of this invention to provide the aforementioned method wherein the container body wall metal has a substantially axial grain direction and its thickness at its marginal open end portion is less than about 0.006 inch for three-piece and less than about 0.0057 inch for two-piece container bodies.
Numerous advantages and objects of this invention will be apparent as it is better understood from the following description, which, taken in conjunction with the drawings, discloses preferred embodiments thereof.